Tanks of the above mentioned type are used for storing liquid components, for example a liquid fuel on the one hand and a liquid oxidizer on the other hand, for operating a spacecraft. A pressurized driving gas is introduced into the tanks to convey or drive the liquid components out of the tanks to the combustion chamber or reaction chamber of the spacecraft in which the fuel and oxidizer will be consumed. Inert gases such as helium (He) or nitrogen (N2) are typically used as driving gases, for which purpose they are introduced under pressure into the fuel tank or the oxidizer tank and thereby press the fuel or the oxidizer into the pipeline system leading from the respective tank to the connected rocket engine. In that regard it is important to achieve a complete, reliable and sure separation between the driving gas used as a conveying medium and the liquid fuel or oxidizer being supplied into the engine, because the liquid fuel and oxidizer must absolutely surely be free of foreign gas inclusions when they are supplied to the engine in order to avoid combustion problems.
When a cryogenic liquid, especially liquid hydrogen, is stored in a tank, the warming of the liquid, e.g. the liquid fuel, over time generally leads to a pressure increase in the tank due to the boiling or vaporizing effect of the cryogenic liquid becoming a gas. The resulting over-pressure must be vented out of the tank upon reaching an upper pressure limit value in order to maintain the structural integrity of the tank. This problematic situation especially arises in cryogenic liquid storage systems for space travel vehicles that must operate over a long time span in orbit in a weightless condition. Over such long time spans, the cryogenic liquids, e.g. fuel and oxidizer, warm up and transition to a gaseous state as described above. In such orbital spacecraft, the excess gas in the tank is then often withdrawn or extracted from the tank and used for position or attitude regulation of the spacecraft. Namely, the cold gas is vented out of the fuel or oxidizer tank and is ejected in a purposefully directed manner through one or more thrust nozzles into the vacuum of space, so as to impart the appropriate positioning or orienting thrust to the spacecraft. Such a system saves costs, complexity, and weight in comparison to the provision of an additional propulsion system for the position and attitude adjustment, and represents a completely adequate variant for thrust generation.
During this process of controlled venting of the tank to generate thrust for position or attitude regulation, if a gas-liquid mixture is extracted from the tank and ejected into the vacuum of space, then the varying densities of the ejected stream of liquid and gas would lead to a varying non-constant thrust profile depending on the mixing ratio. Thus, the regulating algorithm of the thruster system of the spacecraft would then have to correct for these changes or variations of the thrust in view of the mission requirements. Moreover, the ejection of liquid out of the gas extraction arrangement is undesirable insofar as the liquid fuel is then no longer available for fueling the main rocket engine of the spacecraft.
For surely separating the gas and liquid phases in the above context, it has previously been known to carry out the following separation methods in the field of space travel. According to one known method, the mixture of gas and liquid fuel extracted from the tank is heated sufficiently to ensure that any liquid emitted from the fuel tank is vaporized into the gaseous state. Thereby, it is ensured that only gas is supplied to the thruster nozzles. However, this method requires a high amount of energy for vaporizing the liquid fuel. According to a second known method, an additional acceleration is imparted to the spacecraft and thus the fuel tank, so that at the time point of the pressure venting of the tank, the liquid fuel is not located at the gas venting outlet. This method, however, requires an active directed acceleration of the spacecraft by means of an additional propulsion system, which is generally relatively costly and complicates the operation of the spacecraft. Additionally, it is then necessary to adapt the mission profile before carrying out a pressure venting of the tank.
Furthermore it has become known from U.S. Pat. No. 4,027,494 to use phase separators for separating the liquid from the gaseous phase. In this known apparatus a phase separator is used during flight conditions of the spacecraft with only small or minimal acceleration, whereby the separation is carried out by using superconducting magnets. U.S. Pat. No. 4,848,987 further discloses a phase separator that uses pumps and a series of active valves for achieving the phase separation. Still further, U.S. Pat. No. 7,077,885 discloses a phase separator that uses a propeller to set a liquid-gas mixture into rotation, and that comprises a membrane of polyethylene or nylon to separate out the liquid, in this case water. This latter known system is provided for use in connection with fuel cells and is not suitable for separation of cryogenic liquids from gases. Further apparatuses known from U.S. Pat. Nos. 4,435,196 and 4,617,031 are limited to use in the gravitational field of earth, and are thus not suitable for separating cryogenic liquids from gases in tanks in spacecraft.